docs: def process_tree(top, name='top'):
      """Creates a string representation of the process tree for process top.
  
      This method uses the :func:`walk_processes` method to create the process tree.
  
      :param  top:            top process for which process tree string should be
                              created
      :type top:              :class:`~climlab.process.process.Process`
      :param str name:        name of top process
      :returns:               string representation of the process tree
      :rtype:                 str
  
      :Example:
  
          ::
  
              >>> import climlab
              >>> from climlab.utils import walk
  
              >>> model = climlab.EBM()
              >>> proc_tree_str = walk.process_tree(model, name='model')
  
              >>> print proc_tree_str
              model: <class 'climlab.model.ebm.EBM'>
                 diffusion: <class 'climlab.dynamics.diffusion.MeridionalDiffusion'>
                 LW: <class 'climlab.radiation.AplusBT.AplusBT'>
                 albedo: <class 'climlab.surface.albedo.StepFunctionAlbedo'>
                    iceline: <class 'climlab.surface.albedo.Iceline'>
                    cold_albedo: <class 'climlab.surface.albedo.ConstantAlbedo'>
                    warm_albedo: <class 'climlab.surface.albedo.P2Albedo'>
                 insolation: <class 'climlab.radiation.insolation.P2Insolation'>
  
      """
      str1 = ''
      for name, proc, level in walk_processes(top, name, ignoreFlag=True):
          indent = ' ' * 3 * (level)
          str1 += ('{}{}: {}\n'.format(indent, name, type(proc)))
      return str1